1. Field of the Invention
The present invention relates generally to digital audio processing systems, and more specifically, to a digital audio processor including an internal oscillator that provides a reference source that controls an internal intermediate sample frequency of the audio processor.
2. Background of the Invention
Digital audio systems are prevalent in the areas of home entertainment, professional production of multimedia and computer reproduction and generation of multimedia sources. Increasingly, single chip solutions generate and merge multiple streams of audio information. The streams frequently have differing sample rates and may originate as either analog or digital sources. In order to manage such audio processing, a high frequency master clock typically provides the source for various sample clocks that are derived by dividing the master clock. The master clock is typically provided by an external system clock, such as from a crystal oscillator.
Further, the input audio streams are typically provided from different sources having their own reference clock that differ in frequency. Even though the variation in frequency may be slight, due to audio requirements, the streams must still be sample rate converted in order to maintain synchronization over the typically large audio program data lengths. For outputs that must be synchronized to downstream data sinks, the output streams may also require synchronization. For the above reasons, audio processing integrated circuits handling multiple audio streams generally convert all of the incoming audio data to a single intermediate sample rate that is synchronized to the master clock. The output streams are typically generated at the desired output sample rate(s) as derived from the master clock. Use of an intermediate sample rate also eliminates problems with switching synchronization sources. For example, when an audio processor is synchronized to an input sample stream (rather than an independent master clock) and the sample stream clock changes, or is removed, requiring selection/generation of another master clock source, “glitches” in the audio processor outputs will occur.
In some applications, it is desirable to reduce the pin count and the external component count by providing an internal oscillator for an integrated circuit that would ordinarily require an external reference oscillator. A suitable reference oscillator may not be available in the system, or power consumed to distribute a suitably high-frequency reference clock to an integrated circuit may be excessive. Further, electromagnetic interference (EMI) associated with distributing such a reference clock may be unacceptable. While resistor-capacitor (RC) oscillators and inductor-capacitor (LC) oscillators can be implemented within integrated circuits, in audio applications in particular, internal RC oscillators and LC oscillators do not provide sufficient accuracy to serve as digital audio master clock reference sources. In particular, the frequency of such an oscillator will vary with process, voltage and temperature.
Therefore, it would be desirable to provide a digital audio processing integrated circuit with an internal master clock generator that is sufficiently stable and accurate for processing multiple audio input and output streams.